This invention pertains generally to nickel-base superalloys useful in the manufacture of hot-section components of aircraft gas turbine engines, e.g., vanes and rotating blades, and more particularly to compatible coatings especially useful for the enhancement of the environmental resistance of such hot-section components made from advanced nickel-base superalloys and nickel-base eutectic superalloys.
Advanced nickel-base superalloys such as the monocarbide reinforced nickel-base eutectic superalloys of the type described, for example, in U.S. Pat. No. 4,292,076 to Gigliotti, Jr. et al., which is incorporated herein by reference, are designed for use as unidirectionally solidified anisotropic metallic bodies, primarily in the form of vanes and rotating blades in aircraft gas turbine engines. The superalloys of U.S. Pat. No. 4,292,076, when directionally solidified (DS'd) under stringent conditions to achieve planar front solidification (PFS), result in a eutectic composite microstructure consisting of strong, reinforcing metallic carbide (MC) fibers in a .gamma./.gamma.' nickel-base superalloy matrix. Because highly aligned microstructures are formed during planar front solidification, the superalloys of U.S. Pat. No. 4,292,076 offer potential structural stability and property retention to a greater fraction of their solidification temperatures than do other materials.
The eutectic superalloys have been identified as the next generation of blade alloys beyond directionally solidified and single crystal superalloys. In order to take full temperature advantage of those superalloys, however, coatings are required to provide environmental protection at the high intended use temperatures.
Stringent requirements are placed on the coatings and the coating/substrate composite. For example, the coatings must be tightly bonded, i.e., metallurgically bonded, to the substrate and ideally must not degrade either the mechanical properties of the substrate (e.g., ductility, stress rupture strength and resistance to thermal fatigue) or the chemical properties of the substrate (e.g., oxidation resistance and hot corrosion resistance).
Examples of adverse effects to eutectic superalloys which have resulted from incompatible coatings are fiber denudation near the coating/substrate interface due to outward diffusion of carbon from the fiber into the coating and the formation of brittle precipitates in the substrate due to interdiffusion between the coating and the substrate.
While many coatings and barrier/coating systems have been proposed and tried, there has been a general inability in the past to specify coatings or barrier/coating system which are truly compatible with the substrate, i.e., offer improved environmental protection and produce good metallurgical bonds with the substrate yet not degrade the mechanical or chemical propertied of the substrate, especially when the substrate is of an alloy of the type described in U.S. Pat. No. 4,292,076.
Therefore, there exists a need for protective environmental coatings which are truly compatible with the newest generation of superalloys, particularly those designed for use as vanes and rotating blades in aircraft gas turbine engines, such as the directionally solidified monocarbide reinforced nickel-base eutectic superalloys of the type described in U.S. Pat. No. 4,292,076.